For many years the dangers involved with ascending too rapidly following a dive in water have been known. These dangers derive from the action of gases within the body as the body experiences pressure changes.
Air is a mixture of many gases, primarily nitrogen (78%) and oxygen (20%). In a normal ambient conditions, a person inhales air at a normal atmospheric pressure of about 14.7 p.s.i., a portion of the air passing through the lung tissues into the blood. The blood supplies the body cells with oxygen from the air and in turn receives waste carbon dioxide from the cells which it transports to the lung tissues for exhalation. The gases in air when inhaled at normal atmospheric pressure are absorbed by the body.
When air for respiration is supplied to a person in an ambient having a pressure substantially higher than normal atmospheric pressure--such as occurs in an underwater ambient--the air must be supplied at an elevated pressure to offset the pressure of the ambient. The increased pressure results in higher quantities of air, and its constituent gases, being absorbed by the body than would be absorbed at normal atmospheric pressure.
Various types of equipment for supplying air at higher pressure such as self-contained underwater breathing apparatus, or SCUBA) are today readily available and have made possible extended dives in water to great depths. However, the use of such equipment is accompanied by the possibility of hazard.
If inert gases in air, primarily nitrogen, which are dissolved in the body are too quickly released from solution and cannot be safely discharged from the body through the lungs, decompression sickness, or "the bends," occurs. If the ambient pressure to which a person is subjected decreases, inert gases previously dissolved in the blood and body tissues tend to be released from the solution. Such a pressure decrease occurs during ascent from a lower level in a body of water. If the rate of ascent is sufficiently low, the body is able to efficiently discharge the inert gases released from solution and no harm to the body occurs. If the rate of ascent is too high, the body is not able to efficiently discharge these gases (primarily nitrogen) released from solution. As a result, nitrogen gas bubbles form in the body due to the now supersaturated condition of the blood and tissues in which they are located.
These released nitrogen bubbles travel with the blood stream. Should a bubble become lodged in the heart or brain, it can cause death or paralysis. Less serious, but extremely unpleasant and/or painful, are the physiological effects on body tissue of the gasses and the bubbles thereof as they are released from solution. These effects include pain, numbness and muscle weakness. All of the foregoing are often, possibly somewhat imprecisely, referred to as "the bends."
Avoiding "the bends" has been found to be a matter of giving the body sufficient time to discharge through the lungs gases which are released from solution so that bubbles do not form. The body may be afforded this time by appropriate control of the rate of ascent. Ascent at a controlled rate which enables the discharge of nitrogen from the body through the lungs is a part of every experienced diver's basic lore. Ascent at an appropriate rate assures that nitrogen will not be released into or from the blood as bubbles. This appropriate rate may combine controlled ascent with decompression or rest stops during ascent.
Early diving experience with large numbers of divers led the U.S. Navy to produce and circulate its Decompression Tables. These Tables have set the "standard" for ascents at 60 feet per minute. Although dives to great depths may require the earlier noted combination of controlled ascent and rest stops, the U.S. Navy Decompression Tables instruct that ascents from typical depths at rates of 60 feet per minute or less will permit the body to safely discharge gases released from solution to avoid bubble formation and "the bends."
Unfortunately, these Decompression Tables and the 60 feet per minute ascent rate were originally set for male divers aged 19-25 years and in peak physical condition. Recent research indicates that age, sex, physical fitness and individual physiological differences among people all have a bearing on a safe ascent rate, as do depth and duration of dive. The weight of evidence is that a "typical safe" rate of ascent generally applicable is less than 60 feet per minute. Further, there is strong evidence that each person's "safe" rate of ascent is unique and can be arrived at by experience gained from numerous dives and experimentation.
One method of determining one's rate of ascent and for ensuring that a "standard" such as 60 feet per minute is not exceeded, is to carry during the dive a depth guage and a watch. These two measuring devices can be used to periodically calculate rate of ascent. The foregoing technique is inconvenient and subject to error. Two measuring instruments must be carried and nearly simultaneously read and then a calculation must be made. The stress of the dive itself and of events or emergencies--such as low air supply, injury, loss of sense of vertical direction or poor visibility--occurring during the dive create the possibility that the calculations will be erroneously performed, if they are performed at all.
Devices which can accompany a diver and which can alert or inform the diver that a "standard" rate of ascent is being exceeded are known. See, for example the following U.S. Pat. Nos.: 4,820,953; 4,109,140; 4, 107,995; and 3,992,948. Typically, the "standard" is the 60 feet per minute from the U.S. Navy's Dive Tables. In some cases the "standard" may be adjustable to some other ascent rate. The output from the devices of the foregoing patents is of the "go/no-go" variety. Specifically, these prior art devices give the diver no indication of the rate of ascent but merely alert or inform when a pre-set "standard" rate of ascent is being exceeded. By the time the diver receives this information, conditions conducive to "the bends" may have been present for an appreciable time. The diver receives no quantitative data regarding rates of ascent below or above the "standard," and, thus, the diver cannot determine how much or for how long to slow or stop ascent to counteract those effects which may cause "the bends."
U.S. Pat. No. 4,658,358 discloses an underwater computer. This device includes a microprocessor which computes water pressure, depth, minimum depth to which the diver can safely ascent, the minimum time for safe ascent to the water surface, and the elapsed time since the beginning at the dive. The minimum depth and minimum time values are based on an algorithm controlling the operation of the microprocessor. The results of the computation are displayed to the diver. The device does not calculate or display the real time rate of ascent of the diver. Accordingly, if the diver does not "match" the algorithm, the minimum depth and time calculations may be misleading.
Safe diving would seem to require a device which provides a real time indication of the actual rate of ascent and descent of a diver, as opposed to a go/no-go indication that a possibly inapplicable "standard" has been exceeded. Such a device should be easy to use, reliable, easy to read and, preferably, wearable on the diver's body or mounted in a divers console. All of the foregoing are objects of the present invention.
There is some evidence that a diver's rate of descent may play a role in physiological condition during and after a dive. Although the precise role played by descent rate is presently not well defined, a convenient means for ascertaining this value during a dive would be desirable, and the provision of such a means is a further object of the present invention.